The advent of high-quality optical fibers spurred development and installation of communication systems using such fibers to carry light-beam signals between remote points. Such "signals" include binary-encoded data, human speech and TV signals encoded for transmission by light beams. Telephone systems commonly employ optical fibers.
A requirement of such communications systems is that any one of a group of optical fibers can be connected to any one of another group of such fibers. Fiber optic switches perform such a function.
In fact, such switches do not physically connect such fibers. Rather, the fibers (or, more accurately, the signal-carrying cores exposed at the fiber ends) are brought to a "signal-transferring" relationship to one other so that a light beam carried by one fiber is "picked up" by the other.
Two factors are of particular importance in a fiber optic switch. One is that alignment of fibers must be performed very rapidly. And this is so irrespective of whether only two or a relatively large number of fibers is involved. Users have little tolerance for delay occasioned by a switch which is slow to react. Another is that alignment must be performed very accurately since signal attenuation (or avoidance of it) is very much a function of alignment accuracy.
One known approach to fiber alignment involves the use of four photodiodes arranged contiguously one to another in a "2.times.2" arrangement. Such diodes produce an electrical output generally proportional to the amount of light striking the diode. The arrangement uses two or three fibers in side-by-side relationship. One fiber (the center one in a three-fiber configuration) carries the voice or data signal and one or two emit light for alignment. An aligning beam (or beams) of light impinges on the diodes and when the output signal from each diode is generally equal to that of each other diode, the associated signal-carrying fiber is properly aligned.
A disadvantage of this approach is that it seriously affects lens placement and, as a consequence, has an adverse effect on the overall size of the switch.
Other approaches involving fiber alignment (but not in a fiber optic switch) are shown in various U.S. patents, one of which is U.S. Pat. No. 4,677,290 (Mitch). The Mitch patent describes a method for aligning an optical fiber with a single semi-conductor and for fixing (as by soldering) the fiber position, once aligned. The semi-conductor has an electrical characteristic, e.g., voltage, which varies with the intensity of light impinging on it. Light is introduced into the fiber and thence to the semi-conductor and the fiber is moved until the observed electrical characteristic indicates that the desired degree of coupling has been obtained.
The apparatus and method described in U.S. Pat. No. 3,938,895 (Bridger et al.) involves introducing light into one optical fiber which is held stationary. A second fiber is moved until the light coupled into it from the first fiber, as measured by a photodetector, is maximized. The second fiber is then known to be axially aligned with the first fiber.
U.S Pat. No. 4,432,599 (McMahon) shows a fiber optic differential sensor used to determine the displacement of the axis of a light-emitting movable fiber from a "home" or neutral position. The stationary sensing fiber bundle may include two, three or four separate fibers, each of which is connected to a detector and comparator circuit. The movable fiber is known to be in its neutral position when the light received by each of the fibers in the bundle is equal to that received by each of the others. Displacement is measured by detecting imbalances in the received light.
The apparatus shown in U.S. Pat. No. 4,758,061 (Horn) is used for aligning and then joining the ends of two "light waveguides," e.g., fiber optic cables. One waveguide is held stationary and the other is movable in three axes of motion. Light is injected into the stationary waveguide and the movable waveguide is positioned in the "x" and/or "y" directions until the magnitude of injected light received by it is maximized. The waveguides are then known to be in axial alignment. After being so aligned, the movable waveguide is brought toward the stationary waveguide (in the "z" direction) until the ends of the waveguides abut.
The apparatus shown in U.S. Pat. No. 4,792,206 (Skuratovsky) is similar in operation to that shown in the Bridger et al. patent. That is, light is coupled into a fiber optic cable which is held stationary. The end of a movable cable receives light from the stationary cable and is known to be aligned with the stationary cable when the amount of coupled light is at a maximum. Cable movement is by a dual cantilever piezoelectric beam.